Presently, many types of alloys are used in the production of surgical needles. Some such alloys are martensitic stainless steels, austenitic stainless steels, and plated plain carbon steel. These alloys range among materials which exhibit acceptable characteristics regarding corrosion resistance, strength and ductility. Of course, primary among all these factors is strength. Naturally, the ultimate tensile strength of an alloy is ideally as high as possible for use, while not compromising any of the other characteristics of the material. The ultimate tensile strength of the cold drawn precipitation hardening grade steel can be described as a combination of its annealed strength increased by the work hardening response, and added to by precipitation hardening. In general, it is desirable for current chemistries from which needles are formed to have an ultimate tensile strength about equal to 360,000 pounds per square inch (360 ksi), or more.
In general, the alloys on which this application focuses are called maraging stainless steels. This terminology indicates hardening by martensitic transformation, with precipitation hardening by aging. Stainless steel means a relatively high chromium level in the alloy, usually about 12 percent or greater.
The first stage in processing these steels is annealing, or solution treatment. This entails heating the material to a suitable temperature (between 1500.degree. F. and 2100.degree. F.), sufficiently long to place one or more constituent elements into solid solution in the base metal. More preferably, the maraged steels of this invention are solution treated between 1980.degree. F. and 2980.degree. F. The phase change of the solution from an austenitic state to its martensitic state commonly occurs in these alloys during cooling from the elevated temperature of the solution treatment. A rapid cooling rate insures that constituents remain in super saturated solid solution, also avoiding unwanted precipitation that might occur during a slow cool. The transformation to martensite is therefore a diffusionless phase change. Alloy additions remain trapped in solution within the resulting martensite, filling interstitial or substitutional sites of the base metal. In this regard, the additions block dislocation movement and further strain the structural lattice of the alloy. Certain alloy additions may also cause martensite refinement, thus hardening or toughening the alloy due to finer martensite plate spacing.
Next, the alloy is work hardened to gain additional strength. Work hardening is a process which increases the strength of a metal by the addition of mechanical deformation. Any process that increases the resistance to slip or the motion of dislocations in the lattice structure of crystals will increase the strength of the material. In work hardening this resistance is caused by immobile obstacles generated during the deformation process itself. They can be arrays of other dislocations or grain boundaries, the number of which is also increased by the mechanical work.
Finally, precipitation or age hardening is accomplished by aging the alloy at intermediate temperatures, high enough to reactivate both diffusion and the formation of intermetallic compounds. Generally age hardening occurs between temperatures of 750.degree. F. to 1050.degree. F. Typically maraged steels are precipitation hardened between about 825.degree. F. and 975.degree. F. A dispersion of fine precipitates nucleate at dislocations and at martensite plate boundaries, resulting in further hardening of the alloy.
Balancing ultimate tensile strength with corrosion resistance and ductility in maraging steel is difficult to arrange. Many attempts yield high tensile strengths and yet low corrosion resistance, and/or low ductility. Ultimately therefore, it is the goal of this alloy to balance these criteria, in order to produce a strong, ductile and corrosion resistant alloy. Previous systems have attempted to predict the tendency to retain austenite in this regard.
Previously, however, there has been an investigation into alloys in the iron with 12% chromium-system, with variable amounts of nickel, molybdenum and titanium. Previous attempts in predicting the tendency of the steel to retain austenite have been embodied in a number, called the Austenite Retention Index, or ARI. This is seen, for instance, in U.S. Pat. No. 5,000,912 assigned to the common assignee of this invention. There, for some martensitic steels, it has been suggested that the ideal Austenite Retention Index falls between about 17.3% to about 21.4%. However, this index has proved to be inadequate in predicting the amount of austenite which is remaining in the system. Because it is much more valuable to know that the alloy is totally transformed to martensite the use of such indices like the Austenite Retention index do not quite fulfill the requirements of capably producing a useful nickel titanium martensitic steel alloy, which may be useful in making surgical needles.
Another deficiency inherent to the ARI formula is the lack of a capability to predict the amount of Chi phase intermetallic compound in the alloy. When there is increased Chi phase formers in the alloy, this too, results in a loss of ductility. Therefore, this is yet another inadequacy in previous methods of predicting the amount of strength and ductility of the system.
Also, since it is known that the hardening precipitate is a compound containing nickel plus titanium, molybdenum, and tantalum, it is necessary to describe a minimum nickel level to insure adequate hardening.
Of course, once the amount of Chi phase formers, martensitic finish temperature, and minimum nickel level, are derived, it is useful to take these factors and optimize them for amounts of the nickel-titanium-tantalum-molybdenum system percentages so that a final ultimate tensile strength can be predicted. Therefore, a formula to predict ultimate tensile strength based on the amount of these elements present in the alloy would also be useful.